Speech recognition systems translate a speaker's words into digital content or commands. Speech recognition systems are sensitive to their audio input. Background noise and acoustical distortions affect the performance of speech recognition systems. To overcome background noise and acoustical distortions, several techniques are employed. Many large vocabulary speech recognizers require a training process where the user dictates some number of predefined sentences. This training process builds an acoustic model that helps the system to understand not just how the user speaks, but also the background noise and acoustical distortions.